A variety of symmetric (isotropic) and asymmetric (anisotropic) microporous membranes are known. Symmetric membranes have a pore structure (e.g., a mean pore size) that is substantially the same through the membrane from one surface to the other. Asymmetric microporous membranes have a pore structure (e.g., a mean pore size) that varies through the membrane, typically, increasing in size from one surface of the membrane to the other surface. Another type of asymmetric membrane has a pore structure resembling an hourglass shape, decreased at a position within the thickness of the membrane and increased at the opposing surfaces.
However, there is a need for membranes that maintain a desired level of flux for a desired period of time for certain applications. These and other advantages of the present invention will be apparent from the description as set forth below.